This invention relates to microphotonics, and more particularly relates to microphotonic waveguide design and fabrication.
The field of microphotonics has a promising future not only in telecommunications but also in other high-bandwidth information processing applications. Microphotonics is a planar waveguide technology enabling the monolithic integration all of the necessary components for optical computing onto a single microelectronic chip. These components can include lasers, switches, modulators, detectors, and channel add/drop filters, among other photonic and electronic componentry.
This monolithic integration of photonic and electronic circuits holds the key to overcoming bandwidth limitations that are arising in many computation and communication technologies, and the prospects for integrated photonic circuits are immense. Photonic interconnects offer high data-transportation bandwidths with low signal attenuation and virtually zero heat dissipation; and therefore excel where electronic interconnects are limited. Strategic replacement of bandwidth-limited electronic interconnects, such as off-chip memory input/output bus or board-to-board connections in cluster computers, with photonic interconnects can dramatically improve data processing performance. Similarly, the shift towards multi-core architectures also calls for photonic interconnects as high-bandwidth, cross-chip interconnects are implicit in this technology. Photonic interconnects can also be advantageously employed in a chip clock signal distribution system, where reduced jitter, skew, delay, crosstalk and power consumption enabled by the photonic interconnects can all benefit microprocessor performance.
Integration of photonic circuits onto electronic chips can also enable networking technologies that have higher complexity and unique functionality. In the manner of an electronic circuit, an integrated photonic circuit benefits from lower cost, higher reliability, and increased functionality in comparison to linked discrete components. These attributes can benefit and enable network router and transceiver technologies, which currently comprise multiple discrete photonic components, by increasing performance and ultimately reducing cost.
The added microphotonic integration benefits of reduced weight, volume, and power consumption also enable technologies for which these savings can be crucial. The bandwidth-distance limitation of electronic interconnects has conventionally led to a bottleneck in an ability to transport data as fast as it can be processed. The implementation of integrated photonic interconnects holds the promise of bridging microelectronics-based computation technologies with photonics-based communication technologies, thereby eliminating this bottleneck. Ultimately, the unique and cost-effective data processing abilities that accompany integration of photonic circuits with conventional microelectronic circuits can enable the realization of superior communication and computation technologies.
In a manner similar to that of microelectronics, there is a drive for smaller microphotonic devices to enable faster and more complex devices with higher microelectronic wafer yield. Microphotonic device size cannot be arbitrarily reduced, however, because the device size is directly dependent on the index of refraction difference, Δn, between each of the materials included in a system of materials employed for the device. Small photonic devices can be realized when materials having large differences in index of refraction are used.
As the size of photonic devices is reduced and the corresponding index of refraction difference between adjacent device materials is increased, several device challenges arise. For example, the roughness of device surfaces becomes increasingly problematic and results in transmission loss as the index of refraction difference is increased. Roughness arises due to a variety of fabrication conditions in waveguide processing and is conventionally always present. The ability to reduce roughness on the top, bottom, and sidewalls of a microphotonics device such as a waveguide is dependent on the materials system employed for the device.
One well-established microphotonics materials system is the silicon-silicon dioxide materials system. Silicon (Si), typically employed as a waveguide core material, is characterized by a refractive index of about 3.5 at telecommunications wavelengths, and silicon dioxide (SiO2), typically employed as a waveguide cladding material, is characterized by a refractive index of about 1.46 at telecommunication wavelengths. The Si—SiO2 system has been extensively utilized and studied in the microelectronics industry. Not only is the Si—SiO2 system well known, but it has a very high Δn≈2, enabling small and compact microphotonics devices.
Conventionally, a Si waveguide core is preferably fabricated of crystalline Si due to the very low intrinsic bulk transmission loss characteristic of crystalline Si. As a result, in leveraging the high-volume and low-cost of Si-based CMOS processing, the development of a CMOS-compatible integrated photonic circuit technology has resulted in the adoption of single-mode, crystalline Silicon-On-Insulator (SOI) channel waveguides as the optimal waveguide architecture. Unfortunately, the use of crystalline Si waveguides typically confines the complexity of a given photonic circuit to a single level unless expensive wafer bonding fabrication steps are used. For example, epitaxial limitations of SOI-based waveguides constrain their use to a single chip level, which restricts integrated electronic-photonic chip versatility and design freedoms vital to realizing the full potential of an integrated photonic circuit technology. The precision needed for wafer bonding multiple, optically-connected, single crystalline Si layers separated by SiO2 cladding layers has not been reliably demonstrated.
There has been proposed the use of polycrystalline Si, polysilicon as a deposited waveguide core material in addressing the need for a high index-contrast waveguide core material capable of deposition on SiO2 and compliant with the physical and fabrication tolerances utilized in CMOS processing. The use of polysilicon as a waveguide core material offers many of the same benefits of crystalline Si without the restriction on deposition methods. Polysilicon can be precisely deposited by a variety of methods, such as chemical vapor deposition (CVD), sputtering, and E-beam deposition, enabling multiple levels and thus more complex photonic circuits.
Whether crystalline Si, polysilicon, or another material is employed as a waveguide core, it is found that surface smoothing is required to reduce the roughness of device surfaces such that light scattering at all core/cladding interfaces, and the corresponding transmission loss, is minimized. Without surface smoothing, the transmission loss can be sufficiently large to prohibit useful device performance.